Numerical Deconvolution of Surface Interrogation Scanning Electrochemical Microscopy Experiments on Platinum During Hydrogen Evolution

Abstract

It is experimentally challenging to deconvolute the potential‐dependent adsorption of the different intermediates that occur during the hydrogen evolution reaction (HER) in alkaline media. This difficulty has limited our understanding regarding why the HER kinetics are more sluggish in alkaline media compared to acidic media. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI‐SECM) to investigate the surface adsorbed species that form during the HER in alkaline media on polycrystalline platinum, Pt(poly). To deconvolute the different adsorbed intermediates, we developed a detailed COMSOL‐based kinetic model to rapidly simulate the SI‐SECM titration reactions under our experimental conditions. Utilization of this rapid‐kinetic model overcomes the limitation of SI‐SECM not having the ability to simultaneously resolve multiple surface adsorbed intermediates. We demonstrate that these numerical simulations can separate the potential dependant formation of the underpotential deposition of hydrogen (H(UPD)) from the overpotential deposition (H(OPD)) of hydrogen. In addition, our simulations show that a spectator species may also exist on the surface during HER potentials. Our simulations also show that at full H2‐producing potentials, the surface of Pt(poly) is fully saturated with intermediates. Comparison between the potential‐dependent adsorption of H(OPD) and Tafel analysis reveal that the Heyrovsky step is likely rate‐determining in alkaline media. However, in alkaline media the Heyrovsky step transitions from first‐order in H(OPD) at low H(OPD) coverage to zero‐order at high H(OPD) coverage, due to surface saturation of adsorbed intermediates. Tafel analysis in acidic media shows that the Heyrovsky step is likely rate‐determining, but remains first‐order in Had over a larger potential range. These fundamental insights reveal that a sluggish Heyrovsky step is a major contributor to the attenuated kinetics of the HER in alkaline media.

Carbonate-Derived Multi-Metal Catalysts for Electrochemical Water-Splitting at High Current Densities

“…Interestingly, the overpotential for the HER in an alkaline environment is higher than that of the OER presumably due to the sluggish Heyrovsky step, which requires the dissociation of water. 27 Flow-Through Device Characterization. In order to demonstrate proof-of-concept with our highly porous, earthabundant catalyst electrodes, we designed a single stack alkaline electrolyzer (Figure 7).…”

“…The results from the cyclic voltammogram, steady-state, and stability tests all gave similar overpotential values of 355, 372, and 422 mV at 100 mA cm –2 . Interestingly, the overpotential for the HER in an alkaline environment is higher than that of the OER presumably due to the sluggish Heyrovsky step, which requires the dissociation of water …”

“…In alkaline media, the overpotentials for the HER can also significantly contribute to the overall cell voltage. 27 Therefore, a similar approach was applied to developing a lowcost, abundant, and highly active catalyst for the HER. Currently, the best catalysts for HER are platinum-based, a rare and expensive element.…”

“…Finally, we demonstrate the performance of these catalysts in a bench scale flow-through device to achieve overall water-splitting with a high efficiency over a large range of current densities. In alkaline media, the overpotentials for the HER can also significantly contribute to the overall cell voltage . Therefore, a similar approach was applied to developing a low-cost, abundant, and highly active catalyst for the HER.…”

Sulfur incorporation into NiFe oxygen evolution electrocatalysts for improved high current density operation

“…Additional insights into the rate-determining steps can be determined by combining Tafel Slope analysis with the SI-SECM data. 53 As is typically seen with the HER, the Tafel slope will vary depending on which step in the mechanism is rate-determining. Figure 12 shows the Tafel slopes taken from the cyclic voltammetry data for the NiFeS and NiFe catalysts.…”